This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Yellow fever virus (YFV) is a clinically relevant flavivirus and an NIAID Category C Priority Pathogen. YFV is typically spread by mosquitoes and the World Health Organization has estimated that urban yellow fever causes 200,000 cases and 30,000 deaths annually. The current live YFV vaccine used in the United States, YF-VAX[unreadable], has provided significant protection against YFV infection but the risks of vaccination may in some circumstances outweigh the benefits. For instance, immunization with YF-VAX[unreadable] results in 1 to 2 vaccineassociated deaths per million doses administered -including fatalities in young, otherwise healthy adults. Adverse events occur at much higher rates in vulnerable populations such as the elderly, and the live vaccine is formally contraindicated in other vulnerable populations such as infants or individuals with immune disorders such as a history of thymectomy or thymic dysfunction. In this project, we present evidence demonstrating that we have developed a novel vaccine platform that provides an effective and safe alternative to live virus vaccines. We show that virus inactivation using hydrogen peroxide enhances antigenicity and immunogenicity in comparison with other approaches such as formaldehyde-based inactivation techniques. Using this vaccine platform, we propose the development of a new YFV vaccine for use in both healthy individuals and vulnerable populations that have contraindications for immunization with live viral vaccines. This work will involve 1) development of an appropriately rigorous YFV infection model, 2) analysis and optimization of an inactivated YFV vaccine formulation, and 3) characterization of vaccinemediated immune responses and protection against lethal challenge with clinically relevant strains of virulent YFV. The establishment of this second-generation YFV vaccine will represent the first advance in YFV vaccination in 50 years and provide a much needed vaccine alternative for immunologically vulnerable populations.